Lighter-than-air vehicles typically are constructed with inflatable fabric envelopes, filled with lighter-than-air lifting gasses. Such vehicles and can rise up to an altitude of 5,000 meters. Some of them are tethered to the ground by a cable, and many others are self-powered and can move on their own by some kind of propelling system. Lighter-than-air vehicles include aerostats free balloons, airships, and moored balloons. Lighter-than-air vehicles are lifted from the ground aerostatically rather than aerodynamically. They are widely used for telecommunications, radar systems, air traffic management, border control, environment protection and security. Tethered aerostats have the capability of providing continuous and non-interrupted service for 24 hours a day, 7 days a week under all kind of weather conditions.
Lifting gasses for lighter-than-air vehicles can be any gasses that are lighter than air. Hydrogen, helium, methane, ammonia and hot air are all capable of being used for aerostatic lift.
Helium is the most commonly used lifting gas for lighter-than-air vehicles. Helium is the second lightest element on earth. It is called noble gas because it is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas. However, using helium as lifting gas for lighter-than-air vehicles has many drawbacks.
Helium gas leaks from aerostats easily due to its small molecular size. As a result, tethered aerostats need to be refilled periodically to stay up for services. Almost all of today's tethered aerostats have to be brought to the ground for helium replenishment. Therefore, the duration a tethered aerostat can remain airborne are limited by this necessity. Additionally, another concern using helium as lifting gas is that despite its abundance, the supply of helium may be dwindling. Helium is relatively rare element on earth—only 0.00052% by volume of earth's atmosphere.
Hydrogen was once a quite popular lifting gas used for lighter-than-air vehicles. Hydrogen is the first element in the periodic table. It is also the lightest and most abundant element on earth. However, hydrogen is highly flammable with the presence of oxygen that a small spark may cause a hydrogen explosion. Hydrogen was nevertheless widely used as lifting gas because it could be mass-produced through a variety of chemical processes at a low cost.
Methane and ammonia are other alternatives that can be used as lifting gas. Both methane and ammonia are chemical compounds containing atoms of one element bonded with hydrogen atoms. However, they have much less lifting power than either hydrogen or helium. Additional, methane has the drawback of being an explosive compound, and ammonia is dangerous for being toxic.
What is needed is a lighter-than-air vehicle that uses lifting gases that have helium's lifting ability and hydrogen's abundance in supply without the dangerous properties of each of the gases. What is also needed is a means of refilling lighter-than-air vehicle with the lifting gases without having the need to ground the lighter-than-air vehicle.